The present invention pertains generally to producing silicon feedstock for the semiconductor industry, and more specifically, to purifying metallurgical-grade silicon by means of iodine chemical vapor transport to produce pure silicon feedstock for use in fabricating photovoltaic and other semiconductor devices.
About 85% of the photovoltaic modules sold annually are made from silicon. Manufacturers have repeatedly expressed concern about the future supply of low-cost feedstock as this market continues to grow at a rate exceeding 30% each year. Recent reports project that demand for silicon from the electronics industry will exceed the current supply levels by a factor of 2 to 4 within the next decade. This projection does not represent a fundamental material shortage problem because the technology, quartzite, and coke needed to make feedstock are in abundant supply. Rather, the issue is how best to supply the required feedstock with the requisite purity (xcx9c99.999%) to manufacturers at an acceptable cost. Several methods exist for the manufacture of silicon feedstock that meet at least a portion of the manufacturing sector""s requirements, including the widely used silicon chlorosilane method. However, in general, the existing methods are complicated, generate a significant amount of hazardous by-products, require a vacuum system, and are, therefore, quite expensive.
A number of new methods are under consideration for the purification of metallurgical-grade silicon (MG-Si), including: (1) repetitive porous MG-Si etching, gettering and surface-removal of impurities; (2) MG-Si gaseous melt-treatment; and (3) MG-Si purification by recrystallization of Si from MG-Si/metal solutions. Many of these potential methods improve upon the deficiencies of the existing techniques, yet most of the above-referenced techniques still contain some of the above-listed drawbacks, including specifically, the level of complexity of the processes used to generate consistent and predictable results, and which also increase the already high costs associated with producing pure feedstock products. Specifically, the porous silicon etch/gettering removal of impurities, although effective in the near surface region, appears impractical for bulk purification because of the large number of process cycles that would be required and that would thus drive up the time and cost needed to produce purified feedstock in sufficient quantities. The gaseous melt treatment using moist argon appears promising for reducing boron levels from the MG-Si source material, but requires much longer treatment times and more efficient exposure to the liquid silicon in order to be cost-efficient at the level required for this specific problem. Finally, the recrystallization of silicon from MG-Si/metal solutions remains essentially theoretical at this time and is not the short-term solution needed to address current commercial concerns.
Accordingly, an object of the present invention is to is to provide a high deposition rate process for producing pure silicon feedstock from metallurgical-grade silicon.
Another object of the present invention is to provide a viable, economical and high through-put method of depositing pure silicon feedstock for solar cells and other applications.
Yet another object of the present invention is to provide an apparatus by which to produce pure silicon feedstock according to the method of the present invention.
Additional objects, advantages and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the method of this invention may comprise producing pure silicon feedstock by first placing solid metallurgical-grade silicon and solid iodine in the bottom portion of a cold-wall reactor, heating the bottom portion of the cold-wall reactor so as to create a thermal gradient while vaporizing the MG-Si and the iodine, which react chemically to produce SiI2 precursor, drive a portion of the SiI2 to a lower temperature and to thereby deposit the silicon upon a substrate within the cold-wall reactor chamber, and, by taking advantage of a variance in the partial pressures of the metal iodides vapors formed, separate the desirable iodides from the undesirable byproduct iodides by condensation of the desirable iodides on surfaces in the reaction chamber, capturing the iodide condensate in the reaction chamber, and transferring the condensate to a distillation chamber. In the distillation chamber, the condensate of desirable iodides is vaporized, and, once again taking advantage of a variance in the partial pressures of the metal iodide vapors formed to further separate residual undesirable iodide condensates from the desirable SiI4 condensate, collect the SiI4 condensate, and return it to the cold-wall reaction chamber for further cyclical processing until a desired quantity of pure silicon is deposited on the substrate within the cold-wall reactor chamber that it can be removed and replaced with a new substrate.
To produce feedstock using the method described herein, the apparatus of this invention may comprise a plurality of interconnected chambers that are at about atmospheric pressure. A first chamber may have a bottom portion, a mid-portion and a top portion, along with a plurality of inlets and a plurality of outlets. A second chamber may also have a bottom portion, a mid-portion and a top portion, as well as an inlet and a plurality of outlets. A third chamber may have an inlet and an outlet. The metallurgical-grade silicon may be deposited in the first chamber along with an amount of iodine source material. The bottom portion of the first chamber may be heated, thus producing a temperature gradient within the first chamber and also vaporizing a portion of the MG-Si and the I. Some of the vaporized material will form SiI2 which may be deposited upon a substrate in the mid-portion of the first chamber. Additionally, many byproduct metal iodide vapors will be formed, some of which will be separated and removed from the first chamber permanently and some of which will be separated and removed from the first chamber and transferred to the second chamber as liquid condensate.
The second chamber may also be heated, thus producing another temperature gradient and also vaporizing a portion of the liquid condensate. Some of the vaporized condensate will form a SiI4 vapor which will be separated from other metal iodide vapors formed, collected, and transferred to a third chamber to be subsequently returned to the first chamber for re-use. The remaining undesirable metal iodide vapors formed will be separated and removed from the second chamber permanently. Other embodiments and variations based upon the above-described process and apparatus, as well as that which will be disclosed in more detail below, will be recognized by those persons skilled in the art.